Root distribution under seepage-irrigated potatoes in northeast Florida

Much of commercial potato production in Florida is irrigated using sub-surface seepage irrigation. A perched water table is maintained during the season within 50 cm below the top of the potato ridge. Fertilizer placement is critical in this system to maximize plant uptake and to minimize leaching potential. Optimal placement of fertilizers is dependent on root distribution. The objectives of this study were to develop and test a new methodology to spatially describe potato root distribution as affected by nitrogen rate and irrigation system. Soil slices containing representative samples of the potato root system at full flowering were taken from plots fertilized with ammonium nitrate at 168, 224, and 280 kg N ha(-1). The proposed sampling methodology performed satisfactorily. Root length density (cm root cm(-3) soil) and specific root length (cm root mg(-1) root dry weight) were not affected by nitrogen rate, but were affected by spatial position in the soil profile. The highest root length density value (0.72 average) was observed within 12 to 15 cat of the seedpiece. Low root length density values averaging 0.036 were observed between 24 and 36 cm from the top of the ridge. Specific root length values indicated a relatively homogeneous root system in terms of the quantity of invested biomass by unit of root length except in the two central units below 24 cm from the top of the ridge where thickened roots caused significant lower values averaging 6.47 as compared with the average of 15.87 from the surrounding Units in the slice. Root thickening in deep apical roots suggested aerenchyma formation promoted by a combination of saturated soil conditions in the root zone caused by inappropriate irrigation management and soil compaction. Fertilizer placement under the seedpiece should be a good alternative to increase potato nitrogen uptake under seepage irrigation.